The noise data is my measured values which I do several different ways. Some
of which are:
The GPS engine value was calculated from measuring the UNFILTERED RMS noise
of the freq plot data using LadyHeather, backed up by the independent way of
looking at the UNFILTERED 1 sec ADEV values obtained when plotting the ADEV
from that data using an external low noise osc.
The other proof that the data is unfiltered was done by black box testing of
small near instantaneous freq changes of 1e-10 and measuring and how long it
took the Tbolt plot to settle to the new freq value using different filter
setting.
The answer is that it knows the correct freq (within it's nose limits) in
the next 1 sec sample period when the filter is turned off.
As for the ns phase noise that is the RMS Phase noise value from LH using a
good LPRO osc with it's Time constant set to many hrs. (Phase correction TC
was 100K sec). The RMS noise value is very insensitive to the filter setting
up to 1000 seconds because most of the phase noise is slower than 1000
seconds.
As far as the 4 to 10 ns day to day USNO data , that has nothing to do with
sub ns short term noise which I generally limit to more like a few minutes
of sampel time, and if there is a satellite change during the test run, then
I start the test over because I'm looking at GPS engine noise and not the
GPS noise causes by changing satellites etc.
As far as the 4 to 10 ns over a two day period, that agrees pretty well with
what I see some times on a bad day.
On a good day I can get more like 2 to 3 ns, with a 500 sec filter, on a bad
day up to 5 or 6 ns.
For some periods lasting up to 5 to 6 hrs, I've seen numbers as low as 1.5
ns RMS.
ws
******************
)
r
----- Original Message -----
From: "John Ackermann N8UR"
In that test I was just capturing the ADEV table from the TSC-5120 so don't
have raw phase data.
I'm curious where you got the noise data for the TBolt gps engine -- that's
far better than I've seen quoted before. The Trimble data sheet that I
found specs the system PPS accuracy at 20 nanoseconds one sigma; they don't
separately spec the GPS engine. (The data sheet for the current Thunderbolt
E data sheet says 15 nanoseconds.)
The USNO says that their filtered, linear fit time transfer measurements
over a two day period, over the entire constellation, have an RMS residual
of 4 to 10 nanoseconds without SA (http://tycho.usno.navy.mil/gpstt.html).
That may not be apples-to-apples methodology, but it implies that
sub-nanosecond results may be difficult to obtain.
John
----
On Oct 7, 2011, at 4:16 PM, "ws at Yahoo" <warrensjmail-...@yahoo.com>
wrote:
John
What was the RMS and PP phase noise for your 8e-14 test?
Something to keep in mind is that although the Z3801 has a better Osc than
the typical Tbolt.
Long term, really low noise is all about the quality of the GPS signal and
engine.
The Z3801's GPS engine is far inferior and not even close to the 10 to 100
ps, 1 sec noise that a Tbolt engine has.
So how much does that effect the mid term GPS Phase noise which is highly
dominated by how good of antenna system one has.
Sounds like comparison is needed.
ws
*************
Some test like these are on my list for this winter. I am planning to
look at a Z3801A versus a couple of 5061 class units: one with a high
stability tube that I think is in very good condition, and another with an
FTS replacement tube of probably typical surplus quality. I can also add
a TBolt into the mix. I'm hoping to lash up multiple 5334 counters in TIC
mode so I can do several comparisons simultaneously.
In an early test of the 004 tube vs. Z3801A over a week or so, I saw what
looked like a floor in the 8e14 range.
John
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